1. Technical Field of the Invention
This invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing a semiconductor device having a device isolation part.
2. Description of the Related Art
There is an STI (Shallow Trench Isolation) method utilizing a trench structure, as a device isolation art to isolate between the elements on a semiconductor device.
The STI method refers to a scheme of burying an insulation film in a trench formed in a semiconductor substrate and forming a device isolation part thereby providing isolation between elements.
With the STI method, the width of isolation can be reduced as compared to that of the LOCOS (Local Oxidation Of Silicon) scheme utilizing the existing thermal oxidation.
For this reason, attentions are drawn to the STI technique as a micro-fabrication technology for device isolation part due to the recent miniaturization of semiconductor devices (e.g. Patent Document: JP-A-2001-210709).
In the STI scheme, prior to burying a device-isolation insulator within a trench formed in a semiconductor substrate, a thermal oxidation film is formed in an inner surface, as a bottom and sidewall, of the trench. This is because to remove the device damage by defects or the like caused in the trench inner wall by the etching for forming the trench.
However, there are the following problems so far encountered upon forming a thermal oxide film.
Where a thermal oxide film is formed by wet oxidation with heating a substrate at nearly 950° C., the thermal oxide film at a top edge of the trench is reduced in thickness into an acute-angled form (or sharp-edged form). This is because a film stress occurs in the trench top edge when the semiconductor substrate material expands in volume into an oxide film, resulting in lowered oxidation rate at the edge.
As a result, electric-field concentration takes place at the trench top edge, due to which the electric characteristics of transistors are deteriorated by the lowered reliability of the gate oxide film.
Meanwhile, when a thermal oxide film is formed by the dry oxidation with substrate heating at nearly 1100° C., the foregoing problem of reduced thermal oxide film thickness can be overcome. However, (111) crystal plane (facet) occurs mainly at a bottom edge of trench.
Because stresses concentrate at the trench bottom edge due to facet occurrence, crystal defects such as cracks are caused. As a result, the junction leak current of a transistor is ready to occur, which causes deterioration in the transistor electric characteristic.